IMO, there's probably a constellation of contributing factors to how we become who we are. Doubt we need to hit every possible note to get here, and different combinations will probably tip the scales, or not, just the same. Some of them might (probably) be neurological. Some of the neurological factors might not even be sexually dimorphic, but aspects of the personality that leave the door open to it. Innately elevated of cross gender hormones? sure, even if all they do is set you apart from your peers, or give you a glimpse into what it might be like if things were different. I don't think "nurture" alone has much to do with it, but some events or trends might have some impact.
Ultimately, if the gatekeepers are still gatekeeping and they come out with some definitive anatomical test, they would have no legitimacy in my mind if they didn't run those tests on trans folks who had already transitioned before the tests were introduced, to compare statistically if the people who had already crossed over and were happy ALL were found to test positive. If they're content but test negative, there really isn't a problem, and I'd hope the gatekeepers would understand that they clearly aren't testing for everything.
Every single person that lives on this planet has experiences their gender differently than every other person. Same goes for most things we experience. There are plenty of things we have in common, and some things are more common than others, but no one can tell us who we are based on those things. They can only guess.